7.7 Nuclear Fission

A free GCSE Physics lesson for AQA (8463) from OBA School: induced and spontaneous fission, chain reactions, and how a nuclear reactor controls the release of energy. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe what happens when a uranium-235 or plutonium-239 nucleus absorbs a neutron and undergoes fission
• Explain the difference between induced and spontaneous fission
• Explain how a chain reaction happens and the difference between a controlled and an uncontrolled chain reaction
• Describe the jobs of the fuel rods, moderator, control rods, coolant and shielding in a nuclear reactor

Key words

• Nuclear fission: The splitting of a large, unstable nucleus into two smaller nuclei of roughly equal size, releasing energy.
• Induced fission: Fission caused by a nucleus absorbing a neutron.
• Spontaneous fission: Fission that happens without a neutron being absorbed. It is rare.
• Chain reaction: Neutrons from one fission go on to cause more fissions, which release more neutrons, and so on.
• Moderator: A material (such as water or graphite) that slows the fission neutrons down so they can be absorbed by U-235.
• Control rods: Rods (of boron or cadmium) that absorb neutrons. Moving them in or out controls the rate of fission.
• Coolant: A fluid pumped through the core to carry energy away to the heat exchanger.

What happens in fission

Some very large nuclei are fissionable: uranium-235 and plutonium-239 are the ones used in reactors. If a slow neutron is absorbed by a U-235 nucleus, it becomes uranium-236, which is very unstable and almost at once splits into:

• two smaller nuclei (fragments) of roughly equal size, which are radioactive
• two or three neutrons, moving at high speed
• energy, carried away as the kinetic energy of the fragments and neutrons, and as gamma radiation

Worked example: a fission equation

²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n
top: 235 + 1 = 236 and 141 + 92 + 3 × 1 = 236 ✓
bottom: 92 + 0 = 92 and 56 + 36 + 0 = 92 ✓

Answer: mass number and charge are both conserved

Key fact: Fission caused by absorbing a neutron is induced fission. Spontaneous fission happens without a neutron being absorbed, but it is very rare. In both, the nucleus splits and releases neutrons and energy.

Chain reactions

The neutrons released by one fission can be absorbed by other U-235 nuclei, causing more fissions. This is a chain reaction. If every fission led to two more, the number of fissions would double in each step.

• Uncontrolled chain reaction (nuclear weapon): the number of fissions grows very quickly and a huge amount of energy is released in a fraction of a second – an explosion.
• Controlled chain reaction (nuclear reactor): on average exactly one neutron from each fission goes on to cause another fission. Energy is released at a steady rate.

Worked example: counting fissions

Each fission causes 2 more. Step 1 has 1 fission.
step 2: 2, step 3: 4, step 4: 8 … step n: 2ⁿ−¹
step 11: 2¹⁰ = 1024 fissions

Answer: over a thousand fissions after only 10 doublings

Inside a nuclear reactor

• Fuel rods; What it does contain enriched uranium: mostly U-238 with about 3–5% U-235 (natural uranium has less than 1% U-235)
• Moderator (water or graphite); What it does slows down fast fission neutrons by collisions, because slow neutrons are much more likely to be absorbed by U-235
• Control rods (boron); What it does absorb neutrons; lowered further in to slow the reaction or shut it down, raised to speed it up
• Coolant (water under pressure); What it does pumped through the core; carries energy to the heat exchanger where steam is made to drive turbines
• Steel vessel and concrete shield; What it does the thick steel withstands the high temperature and pressure; the concrete absorbs radiation that escapes

U-238 in the fuel does not undergo fission, but it can absorb neutrons and change into plutonium-239, which is fissionable and can be used as fuel in other reactors.

Exam tip: If the control rods are pushed further in, they absorb more neutrons, so fewer fissions happen and less energy is released each second.

How much energy?

Each fission of a U-235 nucleus releases about 3.2 × 10−¹¹ J. That is tiny, but a reactor has an enormous number of fissions every second. Fission releases millions of times more energy per kilogram of fuel than burning coal or gas.

Worked example: fissions per second

A reactor core produces energy at 1.0 GW (1.0 × 10⁹ J per second). Each fission releases 3.2 × 10−¹¹ J.
fissions per second = 1.0 × 10⁹ ÷ 3.2 × 10−¹¹ = 3.1 × 10¹⁹

Answer: about 3.1 × 10¹⁹ fissions every second

Common mistakes

• Mixing up fission (splitting a large nucleus) and fusion (joining small nuclei).
• Saying the moderator absorbs neutrons. The moderator slows them; the control rods absorb them.
• Saying a neutron “hits and smashes” the nucleus. The neutron is absorbed, and the unstable nucleus then splits.
• Forgetting that the fission fragments are radioactive – this is why used fuel is dangerous waste.
• Saying U-238 is the main fissionable isotope. It is U-235 (and Pu-239).

Revision questions

Try these, then check your answers below each one.

Question 1. What must a uranium-235 nucleus absorb for induced fission to happen? (Choose from: a neutron, a proton, an electron, a gamma ray.)

Answer: a neutron

Working: A U-235 nucleus absorbs a (slow) neutron, becomes very unstable and splits.

Question 2. Complete the equation: ²³⁵₉₂U + ¹₀n → ⁹⁰₃₇Rb + ¹⁴⁴₅₅Cs + ? ¹₀n<br/>How many neutrons are released?

Answer: 2

Working: Left: 235 + 1 = 236. Right: 90 + 144 = 234, so 236 − 234 = 2 neutrons. (Bottom: 92 = 37 + 55.)

Question 3. What is the job of the moderator in a nuclear reactor? (Choose from: To slow down neutrons, To absorb neutrons, To carry energy away from the core, To absorb gamma radiation.)

Answer: To slow down neutrons

Working: The moderator (water or graphite) slows neutrons so they are more likely to be absorbed by U-235 and cause fission.

Related lessons

• Previous lesson: 7.6 Nuclear Radiation in Medicine
• Next lesson: 7.8 Nuclear Fusion
• In this chapter: 7.1 Atoms and Radiation
• In this chapter: 7.2 The Discovery of the Nucleus
• In this chapter: 7.3 Changes in the Nucleus

Continue in the full course

This lesson is section 7.7 (AGCPH0707) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.